1. Technical Field
The present invention relates to a process for the preparation of a non-birefringent optical resin suitable for various kinds of optical elements and a optical element made by using the resin prepared by the process.
2. Background Art
For optical elements, such as lenses, prisms, optical disks, LCD (liquid crystal display) substrates and the like, glass has been used hitherto. In recent years, however, a polymeric resin has become to be used in order to make products lighter and more compact. As a polymeric resin used for the optical elements, polystyrene, polycarbonate, polymethyl methacrylate, styrene-methyl methacrylate copolymer and the like are commonly known. However, since polystyrene, polycarbonate and the like have aromatic rings in the molecule, they easily have a birefringence owing to their orientational distortion. Thus, it is necessary to manage to modify a mold as shown in Japanese Published Unexamined Patent Application JP61-14617 A. Therefore, polymethyl methacrylate has been mainly used as the material for the optical elements.
Since polymethyl methacrylate has a small photoelastic coefficient and relatively hardly has a birefringence owing to its orientational distortion, it has been used for optical elements having a relatively low accuracy, such as lenses for finders, pickup lenses for CDs and the like.
In recent years, however, optical elements having higher accuracy has been required. Especially, optical elements, such as laser pickup lenses for write once optical disks and magnetic optical disks, write once optical disks, magnetic optical disks and the like, are required to have not only a small birefringence but also a very small birefringence in the vicinity of the gate.
As a device birefringence of its material comes into question, a liquid crystal device can be cited. As commonly known, a liquid crystal device can rotate the polarization plane of the light in a liquid crystal layer between a polarizer and an analyzer, which are positioned as the crossed Nicole position or the parallel Nicole position, and whereby the liquid crystal device can control the light transmittance. Therefore, with regard to the liquid crystal device, a birefringence of each element makes a big problem and whereby prevents the optical resin from being widely used for the liquid crystal device.
For example, although a birefringence of the methyl methacrylate can be reduced to a certain value by modifying a molding condition, a birefringence in the vicinity of the gate can not be zero. Thus, the methyl methacrylate can not be used for such a optical device of high-accuracy as the liquid crystal display.
Therefore, in order to reduce the birefringence, the following five methods are proposed. (1) Copolymerizing a monomer for a resin having a positive photoelastic coefficient and a monomer for a resin having a negative photoelastic coefficient as the essential raw materials, so that the obtained copolymer has a small absolute photoelastic coefficient equal to or less than 1.times.10.sup.-13 cm.sup.2 /dyne, that is from -1.times.10.sup.-13 cm.sup.2 /dyne to +1.times.10.sup.-13 cm.sup.2 /dyne. (JP60-185236 A) (2) Copolymerizing methyl methacrylate, an alkyl methacrylate having an alkyl group with 3 to 8 carbons and a styrene. (JP60-250010 A and JP60-76509 A) (3) Copolymerizing methyl methacrylate, tricyclo tricyclo[5.2.1.0.sup.2.6 ]deca-8-yl methacrylate and styrene. (JP62-246914 A) (4) Copolymerizing a monomer that can yield a homopolymer having a positive birefringence (trifluoroethyl methacrylate, benzyl methacrylate and the like) and a monomer that can yield a homopolymer having a negative birefringence (methyl methacrylate and the like). (JP2-129211 A) (5) Copolymerizing methyl methacrylate for a resin having a certain photoelastic coefficient and a compound having an unsaturated double bond for a second resin having a photoelastic coefficient whose sign is opposite to that of the first resin. (JP4-76013 A)
Although these conventional methods can obtain the acceptable results, they still have many insufficiencies. For example, the methods of (1), (2), (3) and (5) may not remove a birefringence completely in case of injection molding. Namely, a birefringence owing to stress distortion remains in the vicinity of the gate and the produced resin has an insufficiency as a non-birefringent material.
With regard to the method of (4), although combinations of monomer mixture are disclosed, among them a method of using a monomer mixture of methyl methacrylate (MMA) and trifluoro methacrylate (3FMA) have a problem that the latter (3FMA) is very expensive.
Also, in case of the method of copolymerizing a monomer mixture of methyl methacrylate (MMA) and trifluoroethyl methacrylate (3FMA) or the method of copolymerizing a monomer mixture of methyl methacrylate (MMA) and benzyl methacrylate (BZMA), it is necessary to large the rate of 3FMA/MMA or BZMA/MMA considerably in order to prevent an orientational birefringence being generated. Namely, in case of 3FMA, 50 wt % or more is necessary and in case of BZMA, 20 wt % or more is necessary. Therefore, the methods have a problem that the resin is inferior to PMMA resin in respect of their thermostability (heatstability) and transparency.
The present invention has been achieved to solve the forgoing problems. Thus, the objectives of the present invention are to provide a process for the preparation of a non-birefringent, thermostable and low moisture absorbent optical resin and to provide a non-birefringent, thermostable and low moisture absorbent optical elements made by the resin prepared by the process.